The present invention relates to an electrophotographic photosensitive device essentially composed of a conductive layer, a photoconductive layer and a transparent insulating layer for an electrophotographic copying machine, and to a method of manufacture thereof.
For the purpose of obtaining an electrostatic latent image, there is already known and commercially employed an electrophotographic process using a photosensitive device essentially consisting of a conductive layer, a photoconductive layer and a transparent insulating layer and comprising for example a primary charging step for subjecting the surface of the photosensitive device to a uniform charging of a predetermined polarity, an image exposure step for exposing the surface to a light image, a secondary charging step for applying, simultaneously with the image exposure, an AC corona discharge or DC corona discharge of opposite polarity, and a whole surface exposure step.
One such photosensitive device is formed using aluminium as the conductive layer, selenium as the photoconductive layer and polyethylene terephthalate (PET) as the insulating layer. Since the selenium based photoconductive layer is a P-type material, it is not possible to form electric charge pairs across the insulating layer during the above-mentioned primary charging step of the electrostatic latent image forming process. Therefore, an electrostatic latent image of sufficiently high electrostatic contrast can not be obtained.
Particularly, during the primary charging step, since the dark resistance of the selenium photoconductive layer is high and the injection of carrier from conductive layer side is relatively low, the result is far from the ideal state of forming electric charge pairs only across the insulating layer. Consequently, the intensity of the charging effect of the primary charging is decreased, and the contrast between the bright portions and the dark portions depends only on the simultaneous secondary charging and image exposure steps. When the secondary charging is performed by AC corona charging, substantially no contrast is obtained, and when the secondary charging is DC corona charging of opposite polarity, only a very low contrast can be obtained.
To improve the contrast, an auxiliary method has been proposed wherein the simultaneous secondary charging and image exposure steps are performed after the primary charging step and simultaneously with the whole surface exposure steps or after both the primary charging and whole surface exposure steps. While the contrast obtained by the auxiliary method is improved, it is necessary to add a whole surface exposure lamp to use in whole surface exposure step and the apparatus is, therefore, more complicated. Further, a contrast which is sufficient for practical purposes still cannot be obtained, and in high speed processes, such auxiliary method can not improve the problem.
The prior art has also utilized a photoconductive layer formed by powdered material, e.g. ZnO or CdS, which is scattered in resin binder in the photosensitive device to obtain a high contrast. However, such a photosensitive device involves problem of humidity durability.